The term “analyzer” as used herein refers to a measuring apparatus in process automation engineering that measures certain substance contents with a wet-chemical method, such as, for example, the ion concentration in a medium that is to be analyzed. For that purpose, a sample is taken from the medium that is to be analyzed. Usually, the taking of the sample is performed by the analyzer itself in a fully automated fashion with the help of such means as pumps, hoses, valves, etc. For determining the substance content of a certain species, specific reagents that have been developed for the respective substance content and that are available in the housing of the analyzer are mixed with the sample that is about to be measured. In this way, a color reaction of the mixture is caused that is subsequently measured by an appropriate measuring device, such as, for example, a photometer. To be more precise, sample and reagents are mixed in a cuvette and then optically measured with different wavelengths using the transmitted light method. Thus, the measured value is determined by the receiver based upon light absorption and a deposited calibration model. Typical target measured values are, e.g., ammonia, total phosphate, chemical oxygen demand, and others.
It is extremely important to know the exact amount of different liquids that are being mixed with one another. For a correct determination of the substance content, it is required that the amount of the sample to be measured, as well as the amount of the reagents to be mixed into it, be precisely defined.
One option for measuring a certain quantity of liquid consists in a liquid container with a light barrier.
This method generally shows high accuracy and repeatability, is stable long-term, since the mechanically moved parts do not contribute to the accuracy of dosage, and easily renders itself for automation. As a standard procedure, a dosing tube made of glass is used, with a light barrier (LED and photo detector) for adjusting the correct dosage volume for the respective method attached at the suitable position. A sample or reagent liquid is introduced into the dosing tube via an apparatus, e.g., a piston pump, until the light barrier is triggered. Several light barriers may be attached to a dosing tube, in case several volumes must be dosed.
This method, however, also has several disadvantages: Inappropriate triggering of the light barrier causes erroneous dosings. This happens, for example, in case of drops of liquids that get caught on the dosing tube or move along the dosing tube wall. Liquid membranes that cover the cross-section of the dosing tube like a film may also erroneously trigger the light barrier.
Once arranged, a system with dosing tube and light barrier is very difficult to adapt and/or extend. This is due to the mechanical connection of the light barrier to the dosing tube, as well as the dosing tube cross section selected. In addition, it is impossible to position two light barriers very close to one another, due to the mechanical space demanded by the light barriers. More generally speaking, it is difficult to dose small volumes of liquid and large volumes of liquid in the same system. If the dosing tube is dirtied to a point that it cannot be cleaned anymore, it has to be exchanged. This exchange is often difficult to perform and requires trained staff.